Your selling surface should master product presentation, cart conversion, tax calculation, and order initiation. Keep it fast and opinionated about customer experience, while emitting clean events about orders, refunds, and cancellations. Do not let it mutate accounting entries or inventory truth post‑purchase. If marketing wants flexibility, give them it through metadata, not fragile spreadsheet uploads. Tell us which checkout tweak moved your conversion most, so others can borrow it.
Your ledger must master invoices, payments, adjustments, taxes, and reporting periods. It should accept upstream references, enforce chart‑of‑accounts discipline, and expose reconciled views for cash, revenue, and liabilities. Keep imports structured, automated, and idempotent to avoid duplicates. When disputes arise, the ledger’s audit trail should end arguments quickly. If you are reconciling manually, share what blocks automation, and we will suggest safer import contracts and scheduled jobs.
Assign globally unique, unchanging IDs to orders, customers, items, shipments, invoices, and payouts. Store both external and internal keys, and maintain cross‑reference tables for vendors. Avoid composite keys that include mutable attributes like emails. Use UUIDs or snowflakes and document generation rules. Share which entity causes the most lookup confusion, and we will propose a simple aliasing approach that preserves history without expensive migrations.
Name events with clear verbs and past tense, like order.created or shipment.dispatched, and include timestamps, actors, and source system. Avoid overloading one event to cover multiple scenarios. Emit state transitions, not snapshots, for reliable replays. Keep payloads small but richly referenced. If your queue mixes responsibilities, describe two events that often collide, and we will suggest a minimal split that calms downstream consumers.
Validate inputs at boundaries: schema structure, business constraints, and referential integrity. Reject bad data early with readable errors, log context for triage, and capture samples for test fixtures. Add observability to validation failures and trend them. Periodically replay quarantined events after fixes. Tell us your most frequent validation error, and we will help draft a concise rule that prevents it without choking healthy throughput.

Give different roles distinct permissions for order edits, refunds, and ledger postings. Require approvals for high‑risk actions, and capture reasons. Automate low‑risk paths to speed the majority of work. Periodically review access and revoke stale privileges. If approvals cause delays, share your thresholds, and we will propose a risk‑based matrix that keeps speed where it matters and scrutiny where it protects cash.

Never store raw card data; use tokenization and vaults from certified providers. Minimize personally identifiable information in logs, and mask sensitive fields by default. Document data retention windows and delete on schedule. Keep breach drills realistic and brief. If you are unsure which fields are risky, list your current payloads, and we will highlight easy wins that reduce exposure without breaking necessary analytics.

Adopt append‑only logs or tamper‑evident storage for critical events and approvals. Correlate records across systems with shared identifiers, and provide auditors a read‑only timeline that links orders, shipments, invoices, payments, and journal entries. Automate evidence collection for recurring requests. Tell us what takes longest during audits, and we will recommend a small index or view that collapses hours into minutes.
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